Effect of a Chiral Dopant on the Electro-Optical Properties of Polymer-Dispersed Liquid-Crystal Films Wenbo Li, Hongxue Zhang, Liping Wang, Canbin Ouyang, Xiaokang Ding, Hui Cao, Huai Yang Department of Materials Physics and Chemistry, School of Materials Science and Engineering, University of Science and Technology Beijing, 30 Xueyuan Road, Haidian District, Beijing 100083, People’s Republic of China Received 17 December 2006; accepted 24 January 2007 DOI 10.1002/app.26201 Published online 26 April 2007 in Wiley InterScience (www.interscience.wiley.com). ABSTRACT: Polymer-dispersed liquid crystal (PDLC) films were prepared by the ultraviolet-light-induced poly- merization of photopolymerizable monomers in nematic liq- uid crystal/monomer/chiral dopant composites, and the effect of the chiral dopant on the electro-optical properties of the PDLC films was studied. It was demonstrated that the addition of a small amount of the chiral dopant increased the driving voltage somewhat but decreased the turn-off time significantly. Furthermore, the transmittance of ultraviolet, visible, and near-infrared light of the off state of PDLC films showing light scattering increased with increasing content of the chiral dopant, and the optimum electro-optical properties of the PDLC films were obtained when the content of the chi- ral dopant was not more than 2 wt %. Ó 2007 Wiley Periodi- cals, Inc. J Appl Polym Sci 105: 2185–2189, 2007 Key words: chiral; films; light scattering; optics; photo- polymerization INTRODUCTION Polymer-dispersed liquid crystal (PDLC) films con- sist of a liquid-crystal (LC) material dispersed within a semicontinuous polymer matrix, 1 and they are potentially useful for a variety of electro-optical applications, including switchable windows, flexible large-area displays, and other devices. 2 The films show transparent and light scattering states in the electric field-on and field-off states, respectively. In the electric field-off state, the refractive-index mis- match between the polymer matrix and the LC mate- rial (n p = n LC ) gives rise to the light scattering of incident light. When the LC molecules are aligned in the electric field-on state, the films become transpar- ent because of the matching of the ordinary LC re- fractive index with that of the polymer matrix. 3 The switching properties of the LC molecules within the domains depend on some variables, such as the size and shape of the domains and the molecular interac- tions between the LC material and the polymer ma- trix. The application of an electric field of sufficient strength can overcome the elastic forces anchoring the LC molecules at the interface. Once the electric field is removed, the turn-off time for the molecules to revert back to the equilibrium distribution is also dictated by the strength of these forces. For PDLC films with a low driving voltage, the turn-off time is usually long. 4 In this study, PDLC films were prepared by the ultraviolet (UV)-light-induced polymerization of pho- topolymerizable monomers in nematic LC/photopoly- merizable monomer/chiral dopant composites, and the effects of the chiral dopant on the electro-optical properties of the PDLC films were studied. It was expected that the turn-off time could be decreased by the addition of the chiral dopant. Meanwhile, the var- iation of other electro-optical properties with the addition of the chiral dopant was investigated. EXPERIMENTAL Materials The nematic LC was SLC 7011-100 (Shijiazhuang Yongsheng Huatsing Liquid Crystal Co., Ltd., Shijia- zhuang City, Hebei Province, China), the chiral dopant was CB15 (Merck Co., Ltd., Whitehouse Station, NJ), the monomers were 1,6-bis(allyloxy)hexane (BAOH) and 4-benzoyl-3-hydroxyphenethylacrylate (BHPA; Aldrich Chemical Co., St. Louis, MO), and the photo- initiator was Irgacure 651 (Shijiazhuang Yongsheng Huatsing Liquid Crystal). The chemical structures and some physical properties of the composite films are shown in Scheme 1. Preparation of the PDLC films The following method 5–7 was used for the sample preparation. At first, an SLC 7011-100/BAOH/ BHPA/CB15/photoinitiator composite was prepared. Then, the composite was sandwiched between two pieces of indium tin oxide (ITO), conductive plastic films. After the composite was irradiated by UV Correspondence to: H. Yang (yanghuai@mater.ustb.edu.cn). Contract grant sponsor: Science and Technology Pro- gram of Beijing; contract grant number: Y0405004040121. Journal of Applied Polymer Science, Vol. 105, 2185–2189 (2007) V V C 2007 Wiley Periodicals, Inc.